Optical element for efficient sensing at large angles of incidence

ABSTRACT

An optical element is provided that may be used for collecting optical radiation incident at large angles of incidence. The element may include a grating window formed of grating elements that may collect large incident angle radiation and reflect that radiation, e.g., under total internal reflection in a substantially normal direction. A lens or lens array may be used to collect the internally reflected radiation and focus that radiation to a detector. Example detectors include infrared detectors. The element may be formed to collect, reflect, and redirect optical radiation incident at angles above 45° as measured from a surface normal to a detector oriented substantially at normal incidence. The detected signal from the optical element may be used to control systems within an aircraft or other vehicle capable of flight. In another application, the optical element may also be used to project optical radiation at the same large angles by replacing the detector elements with light sources, such as light emitting diodes.

FIELD OF THE INVENTION

The present disclosure generally relates to optical elements and, moreparticularly, to low profile optical elements capable of collectingoptical radiation at large angles of incidence relative to the surfacenormal of the first optical element and delivering such opticalradiation to an optical sensor located at near-normal incidence relativeto the same first optical surface.

BACKGROUND OF THE RELATED ART

Optical sensors employ optical elements that are used in the collectionof optical radiation and direct such collected optical radiation to anoptical sensor element for measurement of sensor specificcharacteristics based on that collection. To collect such incidentoptical radiation, optical sensors typically use an optical window orlens as the initial optical element, although other optical elements maybe incorporated between the window and the sensor element. A windowgenerally acts as a barrier to protect the following optics from adverseenvironments, such as excess temperature or pressure. The opticalsurfaces of the window are flat and may be parallel, or tilted at aslight angle relative to each other to form an optical wedge. A windowwith parallel optical surfaces does not change the divergence angle ofthe incident optical radiation. An optical wedge will slightly changethe direction of incidence and the divergence angle of the incidentoptical radiation. Additionally, one or both of the optical surfaces ofa lens are curved and act to change the divergence angle of the incidentoptical radiation to effectively direct the incident radiation onto theactive area of the sensor.

Optical elements are most commonly used for collecting axial opticalradiation, i.e., optical radiation incident over a range of angles nearthe axis of the surface normal of the initial optical element. In manyoptical sensor applications, this angular extent is limited by theacceptance angle of conventional optical elements. Such optical elementsare typically limited to an acceptance angle of about 30° (half coneangle). In optical terms this represents a numerical aperture of about0.5, defined as the sine of the half cone angle, or an f number ofapproximately 1, where the diameter of the optical element equals itsfocal length. In some sensor applications, however, it is desirable tohave optical elements that are able to detect non-axial, i.e. off-axisradiation at large angles of incidence, for example from 60 to 80°. Thisis difficult using conventional optical elements.

When optical radiation is incident on an optical window element at largeangles of incidence, although the angle of incidence is reduced insidethe window element, as prescribed by Snell's Law, the exit angle of theoptical radiation is the same as the incident angle. This means thatsensors placed behind a planar window must be oriented so that thesensor element surface normal is parallel to the optical axis of theincident optical radiation. And, in practical sensor applications, suchplacements lead to unacceptably large packaging volumes, especially whenmultiple sensors are employed.

Another problem with the use of conventional optical elements at largeangles of incidence is Fresnel reflection. As is well-known, Fresnelreflections cause a loss of transmitted radiation at transmissivesurfaces due to a reflection from the same surface that increases inmagnitude as the angle of incidence increases. For materials of higherrefractive index, such as semiconductor materials, these Fresnelreflections can result in sizeable losses of optical radiation. As aresult of the foregoing, optical sensor designers are limited in theirability to design effective off-axis optical radiation sensors.

Many types of optical radiation detector have small surface areas, andpractical sensor designs usually require small packaging volumes. Inconventional optical radiation configurations, this requires the use ofa lens with a short focal length. Such lenses are also subject toFresnel losses. Additionally, they introduce focusing errors such aslarge optical aberrations for optical radiation at high angles ofincidence. These aberrations increase as the f number of the lens isdecreased and as the refractive index is increased. This is especiallytrue of semiconductor materials such as silicon or germanium that haveextremely high refractive indices compared to air. These focusing errorscan significantly reduce sensor performance and impact the effectivenessof the overall system to perform its intended function.

Large lenses with short focal lengths have an almost hemisphericalshape. This makes them heavy and difficult to fabricate. Furthermore, inaircraft applications, weight and airflow disturbance considerationsrequire a lens that is similar in profile to the immediate surfacecontour of the external vehicle. To address this, lenses with a facetedstructure, known historically as Fresnel lenses have been used. If suchfacets are configured to be linear, a cylindrical Fresnel lens is formedthat acts in a similar fashion to a conventional cylindrical lens. Acollimated beam incident along the optical axis of the lens is bought toa line focus. If, instead, the facets are configured in a circularpattern, a spherical Fresnel lens is formed that acts in a similarfashion to a conventional spherical lens. A collimated beam incidentalong the optical axis of the lens is bought to a point focus.

These generally flat lenses collect substantially axial radiation, i.e.,radiation over an angular sensitivity range that includes a surfacenormal, and use that collected radiation for imaging or illuminationpurposes. Some have recently proposed Fresnel lenses that include binaryoptical structures formed of discrete steps, which may be formed viaphotomasking techniques. Yet, these Fresnel lens share operation anddesign with the other classes of lens. They are designed for collectionof axial or near axial radiation, not off-axis radiation collection.Other grating structures have been proposed, including diffractiongratings, for example, and large-scale plastic grating structures. Butsuch structures operate by refraction and exhibit some of the alignmentand distortion phenomena discussed above, especially for off-axisradiation.

Others have suggested the use of discrete microprisms for sideillumination in instrument displays. But, the radiation in suchstructures is deflected by a complex arrangement of prisms that makesuniform collection and transmission of light over a sensing area verydifficult. Further, the microprism structures are not periodicstructures.

There have also been suggestions for arrays of microlenses, inapplications such as micro-scale chemical analysis, fiber optic couplingand stereoscopic displays. These are imaging-type optical elements thatoperate by refraction and, as a result, can exhibit losses anddistortions compared to the ideal, desired operation. Non-imagingmicrostructures have also been suggested for light concentration ontodetector arrays using reflective devices, but again the devices aredesigned to collect light incident substantially on axis, not off axis.Plus, these structures do not focus light and may direct light back outof the collection aperture at large angles of incidence.

Combinations of diffraction gratings and lenses have also been proposedto correct for the chromatic aberration of conventional lens inapplications such as photography, image projection and telescopes. Butthere is no capability to collect off-axis radiation, as they arelimited to axial collection.

None of the various known optical elements are able to effectivelycollect off-axis, especially near grazing incidence, light in a low-losssystem. Either from limitations on the angles of the light that can becollected or from the losses that affect that light once collected(e.g., Fresnel reflections), these optical elements present designlimitations to sensor manufacturers.

SUMMARY OF THE INVENTION

An embodiment of the invention is an optical device comprising asubstantially planar grating layer formed of a plurality of gratingelements, each grating element having an entrance face forming a firstangle with a surface normal of the planar grating layer, the entranceface positioned for receiving off-axis radiation, and a reflection faceforming a second angle with the surface normal and positioned to reflectthe off-axis radiation in a direction substantially aligned with thesurface normal and under total internal reflection; at least one lensfor focusing the reflected obliquely incident light onto a focal plane;and at least one photo-detector positioned to receive the reflectedlight at the focal plane.

Another embodiment of the invention is an optical element comprising asubstantially planar grating layer having a plurality of gratingelements disposed to reflect radiation incident upon the grating layerat an off-axis angle of incidence into a substantially normal directionunder total internal reflection; and at least one focusing elementaligned along the substantially normal direction for focusing thereflected radiation to a focal plane.

A further embodiment of the invention includes a method of sensingradiation at an off-axis angle of incidence. The method includes forminga plurality of grating elements having an entrance face and an internalreflection face, wherein the entrance face accepts radiation at theoff-axis angle of incidence and wherein the internal reflection facereflects the radiation in a direction substantially aligned with asurface normal and under total internal reflection; focusing thereflected radiation onto a focal plane; and detecting radiation at thefocal plane.

Another embodiment of the invention is an illumination device comprisinga light source producing a light energy; a lens positioned to collectthe light energy and provide a substantially normal light energy; and asubstantially planar grating layer positioned to receive the collectedlight energy, the substantially planar grating formed of a plurality ofgrating elements, each grating element having a reflection face forminga first angle with a surface normal of the planar grating layer andpositioned to reflect the substantially normal light energy under totalinternal reflection onto an exit face forming a second angle with thesurface normal, wherein the exit face is positioned to provide off-axisradiation.

Some embodiments provide an optical element that may be used in thedetection of radiation incident on a surface at a large angle ofincidence. A grating window may be used for the radiation collection.The size and dimensions of grating elements on that window may beadjusted to direct light collection to a desired range of incidentangles and the elements may be formed to allow for collection of lightfrom different ranges of incident angles, for example by having theelements formed to collect forward- and backward-incident radiation. Thegrating layer may collect light and reflect it under a substantiallylossless total internal reflection to a detector region.

In addition to adjusting grating size and pattern to affect a desiredrange of acceptable incident angles, a lens may be provided on theoptical element, at a size that is suitable to match the sensitivity ofa detector or other analysis tool. Furthermore, different lenses or lenssizes may be employed to create a desired resolution over the detectedarea. Microlenses may be used and patterned in an array form, forexample, to allow for 2-dimensional resolution of the collectedradiation.

The techniques for sensing radiation at large angles of incidence can beused in any number of environments, of which aircraft-based sensing isan example. The optical element may be positioned flush with an aircraftskin, i.e., an outer layer, to detect large angle of incidence radiationnear that skin. For creating a mechanical, electrical, or other responseto such detected radiation, the optical element may be coupled to acontroller capable of controlling connected systems, such asactuator-based flight control systems on an aircraft. Thus the aircraftmay have subsystems responsive to the detection of large angle ofincidence radiation, in an example.

The features, functions, and advantages can be achieved independently invarious embodiments of the present inventions or may be combined in yetother embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A & 1B illustrate an optical sensor module having a large angleof incidence optical element, in accordance with an example.

FIG. 2 illustrates a partial front view of a detailed example of thelarge angle of incidence optical element of FIG. 1.

FIG. 3 an expanded view of example grating elements for the large angleof incidence optical element of FIG. 2.

FIG. 4 illustrates a front view a large angle of incidence opticalelement, in accordance with an example.

FIG. 5 illustrates a front view of a large angle of incidence opticalelement, in accordance with another example.

FIG. 6 illustrates a front view of a large angle of incidence opticalelement, in accordance with yet another example.

FIG. 7 illustrates yet another large angle of incidence optical element,in accordance with an example.

FIG. 8 illustrates a control system in which an optical sensor is usedto control operation of an actuation-based sub-system, as might be usedin an aircraft environment.

FIGS. 9A & 9B illustrate an example environment of use for a large angleof incidence optical element, such as those illustrated in FIGS. 4–7.

FIG. 10 illustrates another optical element in accordance with anexample.

FIGS. 11A and 11B illustrate two different acceptance angle patterns forexample optical elements.

FIG. 12 illustrates an optical element used in an off-axis illuminationdevice.

DETAILED DESCRIPTION OF AN EXAMPLE

Numerous exemplary devices and techniques are described below. Some ofthe devices and techniques are described in relation to somewhatspecific environments of use. The devices and techniques are not limitedto such uses, however, but rather may be implemented in various ways.For example, although some examples are described as usable in anaircraft, the devices and techniques may be used in any number offlight-capable and other vehicles. Furthermore, the devices andtechniques may be used, more broadly, in any environment in whichcollection or projection of off-axis optical radiation may be desired,this may include environments where on-axis radiation collection orprojection is desired as well. Many other examples will be known topersons of ordinary skill in the art upon reviewing the disclosureherein.

FIGS. 1A and 1B illustrate an optical module 100 that may be used tocollect and detect off-axis optical radiation, such as radiation havinga large angle of incidence, for example up to near grazing incidence.The module 100 may be used in an optical sensing application designed toexclusively collect such off axis radiation, or the module 100 may beused as part of a more general sensing application to extend theacceptance angles of incidence beyond just those that are substantiallyaxial. The module 100 may be designed with a small form-factor, thusallowing it to be used in hostile environments, such as at the outersurface of an aircraft, where a material is exposed to extremetemperature and flow conditions.

The module 100 may include a planar optical window 102 resting in ahousing 104 that may be formed of light-weight materials, such asaluminum, titanium, beryllium, or various plastics. As explained infurther detail below, the planar optical window 102 may include a planargrating layer at its exposed surface. These materials are listed by wayof example. In some examples, the housing 104 may be an environmentallysealed enclosure, such as a vacuum chamber.

As shown in cross-section in FIG. 1B. the window 102 may reside in acounterbore 106 extending above a terminus 108. The window 102 may beaffixedly mounted to the terminus 108, via glue mounting, clamping,friction, or the like. In the illustrated example, and as described infurther detail below, the window 102 may include a plurality of lenses110 capable of focusing the collected off-axis radiation. A plurality ofdetectors 112 may be positioned below the lenses 110, in a one-to-oneconfiguration. The detectors 112 may be mounted on a support substrate114 that may provide structural support as well as thermal dissipation.The substrate 114 may be mounted to a floor 116 of the housing 104. Thisconfiguration is by way of example and then only partially illustrated,as the module 100 would provide electrical connection to the detectors112 for driving and/or detecting a sensed characteristic of thecollected radiation.

FIG. 2 illustrates an example optical element 200 that may be used inthe optical module 100 of FIG. 1. The element 200 includes asubstantially planar grating layer 202, which may be formed of materialssuch as zinc selenide, zinc sulfide, calcium fluoride, fused silica,sapphire, germanium, and gallium arsenide. The material should betransparent to the optical radiation in the wavelength range to bedetected. Various plastics, like polycarbonate, acrylic andpolyethylene, may also be used. And persons or ordinary skill in the artwill appreciate that additional materials may be used. Below the planarlayer 202 is a bulk region 204 that may be formed of an opticallytransparent material identical to that of the layer 202. The bulk region204 is optional, however. Abutting the bulk region 204 is a focusinglayer 206, which in the illustrated example, includes a plurality ofcylindrically-extending focusing lenses 208.

Optical coatings may be used between the grating layer 202, bulk region204, and focusing layer 206 to increase transmissivity and minimizeboundary surface reflections. Coatings may also be used for wavelengthregion selection.

In the illustrated example, the layer 202 comprises a plurality ofindividual linear grating elements 210 (only a few bearing referencenumerals), extending along an x-axis. The grating elements 210 can beformed in the bulk optical material used to form the layer 204 or in aseparate optical material. Grating fabrication techniques are known andinclude, laser writing, photomask etching, and diamond cutting, milling,and extrusion. Additional techniques including stamping or rolling froma master grating template. The elements 210 are formed to collectoff-axis radiation, even near grazing incidence radiation, and couplethat radiation through the focusing layer 206 and to a plurality ofdetectors 212 formed on a substrate 214. For example, light rays, 216,218 and 220 are incident upon the element 200 from a large off-axisangle, where the axis referred to is the y-axis as shown, which may alsobe considered a surface normal for the grating layer 204. The light raysmay be incident from any off-axis angle, for example between about 45°and about 90°, i.e., near grazing incidence, as measured from normal.This range is provided by way of example.

This optical radiation (216, 218, and 220) enters the grating layer 202and is turned through an angle larger than 90° so that it passes throughthe bulk region 204 substantially parallel to the y axis. The lens 208then focuses the collected optical radiation 216′, 218′, and 220′ ontothe detector 212. Thus, the optical element 200 as shown is able tocollect light over a range of off-axis incident angles and reflect thatlight in a substantially axial direction, perpendicular to the gratinglayer 202 for focusing onto a detector.

The thickness of the bulk region 204 may be determined by the mechanicalrequirements of the overall sensor structure. Factors include thematerial properties of the optical medium and the pressure differentialbetween the regions above and below the layer 202. For example, if theoptical element 200 is to be used above a vacuum sealed housing, thenthe thickness of the region 204 may be chosen to provide propermechanical strength to withstand the differential pressure.

In the illustrated example, identical lenses 208 and detectors 212 areused, and each detector 212 is positioned a focal length distance fromthe corresponding lens 208. The focal length of the lenses 208 may bedetermined by the radius of curvature and refractive index of theoptical material used to form the lens 206, each of which may beseparately formed and mounted to bulk region 204. The material may beone of the materials identified above and may or may not be identical tothat of the layer 204 or the layer 202. The selection of the radius andfocal length of the lenses 208 may also depend upon design factors suchas the size (along the z axis) of the detector 212 and the desiredacceptance angle of the grating layer 202. Similarly, the width of thelens 208 is largely independent of the size of the grating elements 206,as the lens width is generally much greater than that the width of eachgrating element 210.

The detectors 212 may be visible, ultraviolet, or infrared detectors,for example, depending on the type of radiation to be collected by thewindow 102. Other wavelength ranges may be detected as well. Examplematerials for forming detectors will be known and include silicon,indium antimonide, mercury cadmium telluride, gallium arsenide, indiumgallium arsenide, germanium, indium arsenide, lead sulfide, and leadselenide. The detectors 212 may be in a PIN configuration for example,i.e., is one with a p-type material/insulator/n-type materialconfiguration. Persons of ordinary skill in the art will appreciate thatnumerous detector materials and configurations may be used.

FIG. 3 illustrates a detailed front view of example grating elements 300and 302 (extending into the page) that may be used as the gratingelements 210 of FIG. 2. The grating elements 300 and 302 may collectoptical radiation incident over a range of off-axis incidence angles.The range of acceptable incident angles can vary depending on thegeometry of the elements 300 and 302. Example ranges will now bediscussed for explanation purposes, but persons or ordinary skill in theart will appreciate that geometries may be affected to collect any rangeof off-axis radiation below grazing incidence.

An optimum angle of incidence for a grating layer may be chosen bydetermining the center angle of the desired angular sensitivity range.For example, in FIG. 3, if it is desirable to detect optical radiationincident from 65° to 85°, as measured from the y (normal) axis, 75° isthe chosen center angle. The range may represent Θ_(MIN) and Θ_(MAX),respectively, and the center angle Θ_(C)=(Θ_(MIN)+Θ_(MAX))/2. As aresult, in the illustrated example, a front facet 304, i.e., entranceface, of the first grating element 300 is set to be normal to radiationincident upon the grating layer at 75°. That is, an angle 306 is set to15°, or 90°−Θ_(C). Optical radiation passes through the front face 304into the element 300 and is incident upon a rear facet 308, e.g., areflection face, which serves as a reflection surface.

The rear facet 308 may be angled to turn at least the optical radiationover the desired angular sensitivity range, i.e., between Θ_(MIN) andΘ_(MAX), over 90° so that the collected radiation is propagatingsubstantially parallel to the y-axis. That is, an incident light ray 307is reflected through a reflection angle 309 that is greater than 90°. Inthis manner, the collected light will be directed to a lens (not shown)that may collect light for focusing. To have the center angle radiationreflected off rear facet 308 into a normal direction, a rear facet angle310 is set to half of the desired total turning angle((90°−Θ_(C))+90°)/2), which in the illustrated example is 52.5°. Agrating pitch 312 or a height 314 may then determine the size of eachfacet face 304 and 308, once the angles 306 and 310 have beendetermined. The dimensions may be altered by using different fabricationtechniques or settings and may depend on mechanical considerations ofthe particular environment of use. Grating pitches below 100 μm may beused. For example, the grating pitch may be set to 20 μm in certainapplications.

The grating element 302 may be designed to have identical dimensions tothat of grating 300. Alternatively, the element 300 and 302 may havedifferent dimensions, and in such cases, a grating layer may be formedof a plurality of grating element pairs, each pair formed of a gratingelement 300 adjacent a grating element 302, for periodicity.

The grating device 300 may be designed such that reflections off of therear facet 308 occur under the condition of total internal reflection.Although, the example of FIG. 3 illustrates total internal reflectionfor certain wavelengths from the geometries described above, thesegeometries are examples. The geometry of grating elements 300 and 302 isnot limited to a particular wavelength range, angle, or material. Thegratings may work at infrared, ultraviolet, and visible wavelengths, andall are included within the usage of “radiation” and “optical radiation”herein, along with other wavelengths, as would be appreciated by personsof ordinary skill in the art. The rear facet 308 may be coated with areflective coating, e.g., a metallic or dielectric coating, to increasereflectivity in non total internal reflection geometries or improvereflection over any reflection condition, whether total or partialreflection.

By reflecting off-axis light in a normal or substantially normaldirection, a lens may be used to focus the light onto a small detectoror detector array without detrimental off-axis-induced aberrations andFresnel reflections.

For the lenses in the optical sensor, the lens radius and width ordiameter may be adjusted to match the size, spacing and acceptable angleof the associated detector. Other design factors, such as mechanical andspace constraints may also be used. In an example optical element 400shown in FIG. 4, a radius 402 of lens 404 was set to be twice a width406 of the lens 404, for example, 2 mm in radius and 1 mm in width. Inthis configuration, the lens width is greater than the grating pitch.The radius 402 and width 406 determine the lens sag 408. Additionally,in the illustrated example, the radius 402 and width 404 are set tominimize or reduce total internal reflection at the lens surface 410, asthis would result in Fresnel reflection-type losses for the element 400.

The lens 402 of FIG. 4 is a cylindrical lens. Other lenses may be usedas shown in FIGS. 5–7. FIG. 5 illustrates the front view of an opticalelement 500 with a Fresnel contour lens 502 that may be a spherical lensor cylindrical lens. FIG. 6 is a similar illustration except with anoptical element 600 that includes a lens 602 formed of binary opticalelements 604, 606, 608, and 610, which may be in a ring configuration ona spherical lens, for example. Alternatively, the binary opticalelements 604, 606, 608, and 610 may be in a cylindrical lens form,longitudinally extending into the illustration; By way of example, thebinary optical elements may have step widths below 1 micron and stepheights below 0.5 microns, with the binary optical element 604, 606,608, and 610 having decreasing step widths.

FIG. 7 illustrates another example optical element 700 with an array ofmicrolenses 702. The microlenses 702 may be formed on or separatelymounted to a lower surface 704 of a grating layer 706, which maycomprise a plurality of grating elements 708, such as those describedherein. The microlenses 702 may allow for a smaller-sized opticalmodule. Additionally, with a mircrolens 702 or a circular or ellipticaldiameter lens, in generally, a plurality of detectors may be used in atwo-dimensional array format. This may allow for better spatialresolution of the collected optical radiation by the sensor. And it mayallow for a pixel-based analysis of the radiation collected overdifferent portions of the grating layer 706. Instead of being circularin shape, the microlenses may be made hexagonal, to allow closer packingof the array. Other shapes may be used as well.

Other lenses may be used, including aspherical lenses. Furthermore,while the lenses illustrated above may be used in a cylindrical formextending the length of an axis x, in a spherical form, or an ellipticalform with different curvatures along major and minor axes.

FIG. 8 illustrates an example control system 800 employing an opticalsensor 802 to affect control of other systems, for example,actuator-based systems as might be used on an aircraft. The opticalsensor 802 may be formed of a radiation collector 804, such as thegrating layers described above, and a detector or detector array 806,such as those described above. The sensor 802 may be a component of alarger sensor application, such as multiple off-axis incidence sensors.The sensor 802 may be part of an apparatus capable of collecting lightover a range of acceptance angles including substantially on-axiscollection.

An output from the sensor 802 is coupled to a converter 808, forexample, an analog-to-digital converter (ADC), filter, amplifier, or thelike. The converter 808 is coupled to a controller 810, which may be acentral processing unit or microprocessor capable of accessing memorystorage 812, as shown. Example storage devices include volatile andnon-volatile memory, random access memory (RAM), read only memory (ROM),cache memory, as well as, memory storage media, such as a hard drive, ora CD-ROM or DVD-ROM storage media.

In the illustrated example, the controller 810 controls operation of aplurality of subsystems, Sub1 814 and Sub2 816, in response to the datafrom the sensor 802. For example, if the detector 806 is an arraydetector, then the sensor 802 may provide the controller 810 withmulti-dimensional data on the sensed radiation. The subsystems 814 and816, shown by way of example, may represent actuator based subsystemsthat can convert control signals from the controller 810 into amechanical actuation, such as those used in aircraft and flightapplications. Thus, the single dimension or multi-dimensional data fromthe detector 806 may be used to control different subsystems of anaircraft, including guidance systems.

The controller 810 may provide control signals to the subsystems 814 and816 via bus 818, such as a motherboard bus. The sub-systems 814 and 816may be coupled directly to the bus or coupled thereto via knownconnections, including those compliant with the following standards orprotocols substantially compliant therewith: Institute of Electronicsand Electrical Engineers (IEEE) IEEE-1394b, Universal Serial Bus (USB)1.1, USB 2.0, a Peripheral Component Interconnect (PCI) interface, orother interfacing standard. Additionally, although the subsystems 814and 816 are shown coupled to a single power supply 820, in fact they maybe driven by different power supplies. Additionally, in the illustratedexample, the controller 810 is coupled to an optional second sensor 822for example a sensor capable of measuring operating conditions, such asa temperature sensor. The sensor 822 may be coupled to the opticalsensor 802 for sensing operating conditions within the sensor 822, forexample.

FIGS. 9A and 9B illustrate example applications of an optical sensormodule, where an optical module 900 is disposed in a recess 902 of anexposed covering 904 (partially shown) that may represent a skin on anaircraft. The covering 904 may be have an exposed surface 906 that isflush with a top surface 908 of the optical module 900. The covering 904may be conical in shape, for forming a nose cone of an aircraft orflight vehicle capable of sustaining high-speed flow conditions,including supersonic speeds. The optical module 900 may be sized andpositioned to withstand such fluid flow conditions and still detectoff-axis radiation. The module 900 may be positioned elsewhere on acovering, such as on a fuselage. FIG. 9B shows two modules 900 (notillustrated to scale), one disposed in the covering 904 at a nose end910 of an aircraft 912, the other disposed in the covering 904 at an aftend 914 of the aircraft 912. Although in the illustrated example, themodule 900 is flush with the adjacent outer surface 906, alternativelythe module may extend above or below the surface 906 depending on thedesired angular sensitivity range.

Numerous alternatives are possible. For example, although the opticalelements are described for sensing applications, the detectors may bereplaced with light sources such that the optical elements could be usedas off-axis illumination devices. Expanding light sources would produceoptical radiation collected by lenses that reflect the radiation intooff-axis exit angles by a grating layer. Such devices may be useful foruniform illumination of certain areas, as sources of uniformly directedlight. A strip of light emitting diodes (LED) may be used to generate awhite light, for example, via a red, green, blue LED combination or anultraviolet LED with a fluorescing material. The white light may then bedirected to specific large deflection angle regions using opticalelements as described herein. Applications include indoor and outdoorapplications where light coverage is important.

Further still, the illustrated examples collect light from a particularoff-axis direction, but grating layers may be used to collect light fromdifferent off-axis directions. As shown in FIG. 10, a grating layer maybe formed to collect light from opposing off-axis regions. A gratinglayer 1000 has a first grating region 1002 that may collect radiationover a first acceptance angle range 1004 and a second grating region1006 that may collect radiation over a second acceptable angle range1008. With similar geometries between regions 1002 and 1006, the ranges1004 and 1008 may be identical. The ranges may be different, however, byadjusting the properties of the respective grating. The grating regions1002 and 1006 may having grating elements that extend linearly along anx-axis, or they may represent different sides of a circular gratingelement, meaning that regions 1004 and 1008 are part of a cylindricalcone of acceptable angles. FIG. 11A illustrates an example of the formeracceptance angle pattern, with regions 1004′ and 1008′ for a layer1000′. FIG. 11B illustrates an example of the latter, with regions 1004″and 1008″ for a layer 1000″. In the illustrated example, the gratinglayer 1000 acts as an optical acceptance-angle filter via regions 1004and 1008, and also has an central region 1010 that may be used tocollect on-axis (y-axis) and substantially on-axis radiation, forexample, via a planar window or lens.

FIG. 12 illustrates another embodiment, where the optical path may bereversed by substituting a light source for the detector, or at thefocal plane. An optical element 1100 that may be similar to the opticalelements described above is coupled to receive light from the lightsource 1102, which may be light emitting diodes (LEDs), laser diodes,black body sources such as a filament or arc lamp, or other lightsource. In some examples, the light source 1102 is a side emitting orvertical cavity surface emitting laser. As light diverges from the lightsource, positioned at a focal length of the optical element 1100, thelight is collected by a lens 1104 of the element 1100, which may producea substantially normal and collimated light energy. The lens 110 may be,for example, a cylindrical lens, spherical lens, elliptical lens,Fresnel lens, or binary optical element lens.

The element 1100 has a grating layer 1106, similar to those describedabove, that has a reflection face and an exit face (not shown). The exitface provides off-axis radiation, as shown. Thus, optical radiation fromthe device 1100 may be at large angles from the surface normal of theelement 1100. The radiation may exit the element 1100 at any angle in arange of off-axis angles, such as between about 45° and about 90°, asmeasured from a surface normal. This range is provided by way ofexample.

Specific color (wavelength range) effects may be obtained by usingoptical filters in conjunction with black body sources, or by using LEDsor laser diodes of a specific color, or in combination. In an example,the output from red, green and blue LEDs may be combined by the element1100 to produce white light. That is, each of the LEDs may be positionedbelow the element 1100 in place of the light source 1102. Alternatively,white light LEDs that employ blue/UV radiation to excite a phosphormaterial analogous to a conventional fluorescent tube light may be used.Examples are not limited to visible radiation, but apply to opticalradiation at any wavelength for which a suitable combination of lightsource and optical grating material exist.

Although certain apparatus constructed in accordance with the teachingsof the invention have been described herein, the scope of coverage ofthis patent is not limited thereto. On the contrary, this patent coversall embodiments of the teachings of the invention fairly falling withinthe scope of the appended claims either literally or under the doctrineof equivalents.

1. An optical device comprising: a substantially planar grating layerformed of a plurality of grating elements, each grating element havingan entrance face forming a first angle with a surface normal of theplanar grating layer, the entrance face positioned for receivingoff-axis radiation, and a reflection face forming a second angle withthe surface normal and positioned to reflect the off-axis radiation in adirection substantially aligned with the surface normal and under totalinternal reflection; at least one lens for focusing the reflectedoff-axis radiation onto a focal plane; and at least one photo-detectorpositioned to receive the reflected light at the focal plane.
 2. Theoptical device of claim 1, wherein the substantially planar gratinglayer and the at least one lens abut each other.
 3. The optical deviceof claim 1, wherein the substantially planar grating layer and the atleast one lens are formed of the same material.
 4. The optical device ofclaim 1, wherein the substantially planar grating layer is formed of amaterial selected from the group consisting of zinc selenide, zincsulfide, calcium fluoride, fused silica, sapphire, germanium, andgallium arsenide.
 5. The optical device of claim 1, wherein thesubstantially planar grating layer is formed of a material selected fromthe group consisting of polycarbonate, acrylic, and polyethylene.
 6. Theoptical device of claim 1, wherein the off-axis radiation forms an anglewith the surface normal to the grating layer that is between about 45°and about 90°.
 7. The optical device of claim 6, wherein the angle isbetween about 55° and about 85°.
 8. The optical device of claim 1,further comprising a reflective layer disposed on an exposed side of thereflection face.
 9. The optical device of claim 1, wherein the at leastone lens comprises a cylindrical lens having a longitudinal axisparallel to a longitudinal axis of the plurality of grating elements.10. The optical device of claim 1, wherein the at least one lenscomprises a spherical lens.
 11. The optical device of claim 1, whereinthe at least one lens comprises an elliptical lens.
 12. The opticaldevice of claim 1, wherein the at least one lens comprises a Fresnellens.
 13. The optical device of claim 1, wherein the at least one lenscomprises a binary optical element lens.
 14. The optical device of claim1, wherein the at least one photo-detector comprises an infrareddetector.
 15. The optical device of claim 14, further including adetector array comprising the at least one photo-detector.
 16. Theoptical device of claim 14, wherein the infrared detector is formed ofindium antimonide.
 17. The optical device of claim 14, wherein theinfrared detector is formed of mercury cadmium telluride.
 18. Theoptical device of claim 1, wherein the reflection face is disposed toreflect the off-axis radiation more than 90°.
 19. An optical elementcomprising: a substantially planar grating layer having a plurality ofgrating elements disposed to reflect radiation incident upon the gratinglayer at an off-axis angle of incidence into a substantially normaldirection under total internal reflection; and a plurality of focusingelements aligned along the substantially normal direction for focusingthe reflected radiation to a focal plane as a plurality of focusedradiation beams.
 20. The optical element of claim 19, wherein each ofthe plurality of grating elements comprises an entrance face and aninternal reflection face, wherein the internal reflection face reflectsthe radiation incident upon the grating layer at the off-axis angle ofincidence a reflection angle greater than 90°.
 21. The optical elementof claim 20, wherein the entrance face and internal reflection face aredisposed to reflect radiation incident within, a range of off-axisangles of incidence into the substantially normal direction under totalinternal reflection.
 22. The optical element of claim 19, wherein theoff-axis angle of incidence is greater than about 45°.
 23. The opticalelement of claim 19, wherein the focusing element is a convex lens. 24.The optical element of claim 19, wherein the focusing element is aFresnel lens or a binary-optical element lens.
 25. The optical elementof claim 19, wherein the plurality of focusing elements comprise anarray of focusing elements.
 26. An optical module comprising: a housinghaving an outer surface; an optical element disposed within the housing,the optical element comprising a substantially planar grating layerdisposed adjacent the outer surface and having a plurality of gratingelements disposed to reflect radiation incident upon the grating layerat an off-axis angle of incidence into a substantially normal directionunder total internal reflection, and the optical element having at leastone focusing element aligned along the substantially normal directionfor focusing the reflected radiation to a focal plane; and at least onephotodetector disposed within the housing to detect radiation of thefocal plane.
 27. The optical module of claim 26, wherein the at leastone photodetector comprises an array of photodetectors.
 28. For use in avehicle capable of flight, an apparatus comprising: an exposed skindefining a recess; and an optical module disposed within the recess ofthe exposed skin for detecting radiation over an off-axis range ofincident angles, as measured from an axis defined by the exposed skin,the optical element having a plurality of grating elements disposed toreflect radiation incident upon the grating layer at an off-axis angleof incidence into a substantially normal direction under total internalreflection, and the optical element having at least one focusing elementaligned along the substantially normal direction for focusing thereflected radiation to a focal plane.
 29. A method of sensing radiationat an off-axis angle of incidence, the method comprising: forming aplurality of grating elements having an entrance face and an internalreflection face, wherein the entrance face accepts radiation at theoff-axis angle of incidence and wherein the internal reflection facereflects the radiation in a direction substantially aligned with asurface normal and under total internal reflection; focusing thereflected radiation onto a focal plane; and detecting radiation at thefocal plane.
 30. The method of claim 29, further comprising forming theplurality of grating elements to reflect radiation incident at an angleof between about 85° and about 55°, as measured from the surface normal.31. The method of claim 29, further comprising: disposing a plurality oflenses to focus the reflected radiation to different locations on thefocal plane; and disposing a plurality of detectors at the differentlocations.
 32. The method of claim 31, wherein the lenses are sphericallenses.
 33. The method of claim 31, wherein the lenses are ellipticallenses.
 34. The method of claim 31, wherein the lenses are Fresnellenses.
 35. The method of claim 30, wherein the lenses are binaryoptical element lenses.
 36. The method of claim 29, wherein the detectoris an infrared detector.
 37. The method of claim 36, wherein theinfrared detector is formed of indium antimonide.
 38. The method ofclaim 36, wherein the infrared detector is formed of mercury cadmiumtelluride.
 39. The method of claim 29, further comprising reflecting theradiation more than 90°.
 40. An illumination device comprising: a lightsource producing a light energy; a lens positioned to collect the lightenergy and provide a substantially normal light energy; and asubstantially planar grating layer positioned to receive the collectedlight energy, the substantially planar grating formed of a plurality ofgrating elements, each grating element having a reflection face forminga first angle with a surface normal of the planar grating layer andpositioned to reflect the substantially normal light energy under totalinternal reflection onto an exit face forming a second angle with thesurface normal, wherein the exit face is positioned to provide off-axisradiation.
 41. The illumination device of claim 40, wherein the lightsource is a light emitting diode.
 42. The illumination device of claim40, wherein the light source is a laser source.
 43. The illuminationdevice of claim 40, wherein the substantially planar grating layer andthe lens are formed of the same material.
 44. The illumination device ofclaim 40, wherein the substantially planar grating layer is formed of amaterial selected from the group consisting of zinc selenide, zincsulfide, calcium fluoride, fused silica, sapphire, germanium, andgallium arsenide.
 45. The illumination device of claim 40, wherein thesubstantially planar grating layer is formed of a material selected fromthe group consisting of polycarbonate, acrylic, and polyethylene. 46.The illumination device of claim 40, wherein the off-axis radiationforms an angle with the surface normal to the grating layer that isbetween about 45° and about 90°.
 47. The illumination device of claim46, wherein the angle is between about 55° and about 85°.
 48. Theillumination device of claim 40, wherein the lens comprises acylindrical lens having a longitudinal axis parallel to a longitudinalaxis of the plurality of grating elements.
 49. The illumination deviceof claim 40, wherein the lens comprises a spherical lens.
 50. Theillumination device of claim 40, wherein the lens comprises anelliptical lens.
 51. The illumination device of claim 40, wherein thelens comprises a Fresnel lens.
 52. The illumination device of claim 40,wherein the lens comprises a binary optical element lens.
 53. Theillumination device of claim 40, wherein the reflection face is disposedto reflect the substantially normal light energy more than 90°.
 54. Amethod of providing radiation at an off-axis angle of incidence, themethod comprising: providing a light source producing a light energy;collecting the light energy; propagating the light energy in asubstantially normal direction; disposing a plurality of gratingelements to receive the light propagating in the substantially normaldirection, each grating element having an internal reflection face thatreflects the light propagating in the substantially normal directionunder total internal reflection, and each grating element having an exitface for producing radiation at the off-axis angle of incidence.
 55. Themethod of claim 54, further comprising disposing the plurality ofgrating elements to reflect radiation incident at an angle of betweenabout 85° and about 55°, as measured from a surface normal.
 56. Themethod of claim 54, further comprising disposing a spherical lens tocollect the light energy.
 57. The method of claim 54, further comprisingdisposing an elliptical lens to collect the light energy.
 58. The methodof claim 54, further comprising disposing a Fresnel lens to collect thelight energy.
 59. The method of claim 54, further comprising disposing abinary optical element lens to collect the light energy.
 60. The methodof claim 54, further comprising reflecting the light propagating in asubstantially normal direction greater than 90°.